The present invention generally relates to pipeline cracks and more particularly to arresting ductile propagating fractures without “ring-off”.
Axial crack arrest capability is one design consideration for pipelines (or simply “pipes”) containing and transporting high-energy fluids. A “high-energy fluid” is one that does not decompress quickly, such as natural gas, rich natural gas with heavier hydrocarbon additives, or liquid CO2. A pipeline with oil or water in it will decompress rapidly and a crack would quickly arrest in such a case.
Once an axial fracture initiates, it can propagate in a brittle manner or in a ductile manner. Modern pipeline steels can be designed readily to avoid a brittle fracture, which propagate along the pipe length at about 1,500 feet/second or greater. However, ductile propagating fractures, which propagate between about 300 and about 1,200 feet/second in high-energy pipelines, are more difficult to control. The most common causes of such propagating fractures are corrosion and third-party damage to the pipeline from, for example, excavation or construction equipment.
Crack arrestors are designed and installed on pipelines to restrict uncontrolled propagating ductile fractures down the length of the pipeline. These crack-arrestors usually are mechanical devices installed on the pipeline at regular spaced intervals to arrest ductile fracture instantaneously upon encountering this device.
A variety of crack arrestors have been proposed:    Wilkowski, G., Scott, P., and Maxey, W., “Design and Optimization of Mechanical Crack Arrestors for Pipelines,” NG-18 Report 134, July 1983    U.S. Pat. No. 4,327,473 4,559,974 4,676,276 4,700,752 4,195,669 4,180,104 4,176,691 4,148,127 4,224,966 4,284,107
Most of these crack arrestors are designed to stop further propagation without consideration to how the crack is arrested, i.e., whether the pipe is thrown out of the ditch during the fracture arrest event or stopped within the initial construction ditch and right-of-way of the pipeline. Frequently, crack arrest involves a full-bore opening of the pipeline when a propagating axial crack turns in the circumferential direction at the edge of the arrestor and propagates around the circumference to create a guillotine break in the pipeline. Such arrest behavior also is termed as a “ring-off” and leads to the complete severance of the pipeline. The “ring-off” behavior at the arrestor causes sections of the pipeline to be ejected from the ditch in which it was buried. In full-scale testing, large-diameter pipe sections have been thrown up to ¼-mile from the initial construction ditch when such “ring-off” arrests occurred. This distance is well outside the typical right of way of the pipeline, not to mention quite dangerous to personnel and property in the area.